Supermicrocellular foamed materials

ABSTRACT

A supermicrocellular foamed material and a method for producing such material, the material to be foamed such as a polymerplastic material, having a supercritical fluid, such as carbon dioxide in its supercritical state, introduced into the material to form a foamed fluid/material system having a plurality of cells distributed substantially throughout the material. Cell densities lying in a range from about 10 9  to about 10 15  per cubic centimeter of the material can be achieved with the average cell sizes being at least less than 2.0 microns and preferably in a range from about 0.1 micron to about 1.0 micron.

This is a divisional of copending application Ser. No. 07/682,116 filedon Apr. 5, 1991, now U.S. Pat. No. 5,158,986.

INTRODUCTION

This invention relates generally to foamed materials, preferably foamedplastic materials, and to techniques for making and using suchmaterials, and, more particularly, to the use of supercritical fluidsfor producing supermicrocellular foamed materials which can achieve arelatively wide range of material densities and a large number ofextremely small voids or cells per unit volume therein.

BACKGROUND OF THE INVENTION

Techniques for making conventional foamed materials, such as foamedpolymer plastic materials, have been well known for many years. Standardtechniques for such purpose normally use chemical or physical blowingagents. The use of chemical agents is described, for example, byLacallade in the text, "Plastics Engineering," Vol. 32, June 1976 whichdiscusses various chemical blowing agents, which agents are generallylow molecular weight organic compounds which decompose at a criticaltemperature and release a gas (or gases) such as nitrogen, carbondioxide, or carbon monoxide. Techniques using physical agents includethe introduction of a gas as a component of a polymer charge or theintroduction of gases under pressure into molten polymer. Injection of agas into a flowing stream of molten plastic is described, for example,in U.S. Pat. No. 3,796,779 issued to Greenberg on Mar. 12, 1976. Suchearlier used and standard foaming processes produce voids or cellswithin the plastic materials which are relatively large, e.g., on theorder of 100 microns, or greater, as well as relatively wide ranges ofvoid fraction percentages e.g., from 20%-90% of the parent material. Thenumber of voids per unit volume is relatively low and often there is agenerally non-uniform distribution of such cells throughout the foamedmaterial. Such materials tend to have relatively low mechanicalstrengths and toughness and there is an inability to control thedielectric constant thereof.

In order to improve the mechanical properties of such standard cellularfoamed materials, a microcellular process was developed formanufacturing foamed plastics having greater cell densities and smallercell sizes. Such a process is described, for example, in U.S. Pat. No.4,473,665 issued on Sep. 25, 1984 to J. E. Martini-Vredensky et al. Theimproved technique provides for presaturating the plastic material to beprocessed with a uniform concentration of a gas under pressure and theprovision of a sudden induction of thermodynamic instability in order tonucleate a large number of cells. For example, the material ispresaturated with the gas and maintained under pressure at its glasstransition temperature. The material is suddenly exposed to a lowpressure to nucleate cells and promote cell growth to a desired size,depending on the desired final density, thereby producing a foamedmaterial having microcellular voids, or cells, therein. The material isthen quickly further cooled, or quenched, to maintain the microcellularstructure.

Such a technique tends to increase the cell density, i.e., the number ofcells per unit volume of the parent material, and to produce muchsmaller cell sizes than those in standard cellular structures. Themicrocellular process described tends to provide cell sizes that aregenerally smaller than the critical sizes of flaws that preexist inpolymers so that the densities and the mechanical properties of thematerials involved can be controlled without sacrificing the mechanicalproperties of some polymers, such as the mechanical strength andtoughness of the polymer. The resulting microcellular foamed materialsthat are produced, using various thermoplastics and thermosettingplastics, tend to have average cell sizes in the range of 3 to 10microns, with void fractions of up to 50% of the total volume andmaximum cell densities of about one billion (10⁹) voids per cubiccentimeter of the parent material.

Further work in producing microcellular foamed plastic material isdescribed in U.S. Pat. No. 4,761,256 issued on Aug. 2, 1988 toHardenbrook et al. As set forth therein, a web of plastic material isimpregnated with an inert gas and the gas is diffused out of the web ina controlled manner. The web is reheated at a foaming station to inducefoaming, the temperature and duration of the foaming process beingcontrolled prior to the generation of the web to produce the desiredcharacteristics. The process is designed to provide for production offoamed plastic web materials in a continuous manner. The cell sizes inthe foamed material appear to lie within a range from 2 to 9 microns indiameter.

It is desirable to obtain improved foamed materials which will provideeven smaller cell sizes, e.g., 1.0 micron or less, and much higher celldensities as high as several thousand trillions of voids per cubiccentimeter, i.e., on the order of 10¹⁵, or so, voids per cubiccentimeter of the parent material, for example. Such materials shouldalso have a capability of providing a wide range of void fractionpercentages from very high void fractions (low material densities) up to90%, or more, to very low void fractions (high material densities) downto 20%, or less.

Further, it is desirable to be able to produce microcellular plastics ator near ambient temperature, so as to eliminate the need to heat theplastic during the process thereby simplifying the manufacturingprocess. Moreover, it is further desirable to increase the speed atwhich a fluid is dissolved in a polymer so that the overall time of thefoaming process can be significantly reduced so as to increase the rateof production of the foamed material.

No processes used or proposed for use to date have been able to providefoamed materials having such extremely small cell sizes, such extremelyhigh cell densities and such a wide range of material densities thatprovide improved material characteristics. Nor have techniques beenproposed to obtain such materials at ambient temperature and atincreased production rates.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention, supermicrocellular foamed materialsare formed by using supercritical fluids, i.e., gases in theirsupercritical state, which supercritical fluids are supplied to thematerials to be foamed. The supercritical fluid is used as the foamingagent in a parent material, preferably, for example, in a polymerplastic material. A relatively high density supercritical fluid made ata relatively low temperature and a relatively high pressure is used tosaturate the polymer without the need to raise the saturationtemperature of the process to the melting point of the polymer.

While the mechanism for achieving saturation is not fully understood indetail, it is believed that the supercritical fluid (as a solute) isinitially dissolved in the polymer material (as a solvent) until theconcentration percentage of supercritical fluid in the polymer reaches areasonable level, e.g., perhaps about 10% to 40%. At some percentagelevel then, it is believed that supercritical fluid then tends to act asa solvent and the polymer tends to act as a solute. However, whether thesupercritical fluid and polymer act as solvents or solutes during theprocess, at some time following the introduction of supercritical fluidinto the polymer, an effectively saturated solution of the fluid and thepolymer is produced. Although the aforesaid description is believed tobe a reasonable theoretical explanation of what occurs during theprocess involved, the invention is not be construed as requiring thatsuch specific process necessarily occurs in the manner so described.

When the fluid/polymer solution contains a sufficient amount ofsupercritical fluid therein at a suitably selected temperature andpressure, the temperature and/or pressure of the fluid/polymer system israpidly changed to induce a thermodynamic instability and a foamedpolymer is produced. The resulting foamed material can achieve a celldensity of several hundred trillions of voids per cubic centimeter andaverage void or cell sizes of less than 1.0 micron, in some cases lessthan 0.1 micron. Moreover, in accordance with the invention, the foamingof such materials can in some cases be achieved at ambient (room)temperature conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be described in more detail with the help of thefollowing drawings wherein

FIGS. 1 and 1A depict graphs of the pressure vs. specific volumerelationship showing the region in which a supercritical state isachieved for carbon dioxide;

FIG. 1B depicts a graph of the pressure vs. temperature relationshipshowing the region in which a supercritical state is achieved for carbondioxide;

FIG. 2 depicts a chart of critical temperatures and critical pressuresrequired for placing various materials in their supercritical fluidstates;

FIG. 3 depicts a diagrammatic view of an exemplary system for formingsupermicrocellular foamed materials in accordance with the invention;

FIGS. 4 and 5 depict graphs of pressure vs. volume relationships helpfulin understanding an exemplary method of the invention for an ideal andfor an actual operation of the system of FIG. 3 when using carbondioxide;

FIGS. 6, 7, 8, 9, 10, and 11 depict microphotographs showing typicalcross-section views of the cells produced in various supermicrocellularfoamed materials in accordance with the invention (note the micronscales indicated in the figures);

FIG. 12 depicts bar graphs of the average cell sizes produced forvarious supermicrocellular foamed polymer plastic materials formed inaccordance with the invention under substantially the same exemplaryconditions;

FIG. 13 depicts bar graphs of the average cell densities produced forthe various supermicrocellular foamed polymer plastic materials shown inFIG. 12 formed under substantially the same exemplary conditions;

FIG. 14 depicts a continuous system using extrusion techniques forproviding sheets of foamed materials in accordance with the invention;

FIG. 15 depicts an alternative continuous system using extrusiontechniques in accordance with the invention;

FIG. 16 depicts a further alternative continuous system using extrusiontechniques in accordance with the invention;

FIG. 17 depicts a system in which the foaming of a material and theforming of an article therefrom can take place in accordance with theinvention; and

FIG. 18 depicts an injection molding system in accordance with theinvention in which the time required for saturation of a material by asupercritical fluid is greatly reduced from that normally required formicrocellular foaming.

A supercritical fluid can be defined as a material which is maintainedat a temperature which exceeds a critical temperature and at a pressurewhich exceeds a critical pressure so as to place the material in asupercritical fluid state. In such state, the supercritical fluid hasproperties which cause it to act, in effect, as both a gas and a liquid.Thus, in the superciritical state, such a fluid has the solventcharacteristics of a liquid, but the surface tension thereof issubstantially less than that of a liquid so that the fluid can diffusemuch more readily into a solute material, as in the nature of a gas.

For example, it is known that carbon dioxide (CO₂) can be placed in asupercritical state when its temperature exceeds 31° C. and its pressureexceeds 1100 psi. FIGS. 1 and 1A, for example, depict curves 10 and 12of pressure vs. specific volume (FIG. 1) and temperature vs. specificentropy (FIG. 1A) for carbon dioxide. When the pressure is above 1100psi and the temperature is above 31° C. (curve 10A) exemplified by theshaded region 11 of FIG. 1, and when the temperature is above about 31°C. and the pressure is above 1100 psi (curve 12A) exemplified by theshaded region 13 of FIG. 1A, carbon dioxide is provided in itssupercritical state. As depicted another way, FIG. 1B shows the pressurevs. temperature relationship for CO₂ in which such critical pressure(1100 psi) and critical temperature (31° C.) are depicted so as todefine the supercritical state by the shaded region 14.

The chart of FIG. 2 depicts the critical temperatures and pressures forvarious known exemplary materials, above which values such materials areplaced in their supercritical fluid states.

FIG. 3 shows a diagrammatic view of an exemplary system for use informing supercritical foamed materials in accordance with the invention.As can be seen therein, a source of carbon dioxide in a non-criticalstate is provided from a pressurized CO₂ cylinder 20 in which CO₂ ismaintained at a pressure and temperature below the above discussedcritical values. The CO₂ therein is supplied through conduit 21 via ahigh-pressure valve 22 to a high pressure chamber 23. The temperature ofthe chamber can be controlled, for example, by placing the chamber in atemperature controlled enclosure 24. A material 25, such as a polymerplastic material, is placed within chamber 23. The temperature of thechamber is controlled to be set at a selected initial temperature level.

In order to understand the process of the invention for providing asupercritical fluid, such as CO₂ in its supercritical state, to chamber23 for use in producing a foamed material, it is helpful to consider thepressure-volume relationships shown in FIGS. 4 and 5 which depict suchrelationships both ideally (FIG. 4) and in an actual experimental case(FIGS. 5) when CO₂ is used in its supercritical fluid state with a softpolyvinyl chloride polymer plastic.

In accordance with a specific exemplary process for providing asupercritical CO₂ fluid, the temperature of chamber 23 is initially setat 25° C., via a suitable temperature control of enclosure 24 usingcontrol techniques as would be well-known to those in the art. A CO₂ gasis maintained in cylinder 20 at a pressure of 850 psi (5.8 MPa), forexample, and high pressure valve 22 is opened to supply CO₂ gas at suchpressure to chamber 23 via conduit 21. Valve 22 is closed (point A ofFIGS. 4 and 5) so that initial conditions of a temperature of 25° C. anda pressure of 850 psi are established in chamber 24.

The temperature of chamber 24 is then reduced to 0° C. at which pointthe pressure drops to 515 psi (point B of FIGS. 4 and 5). The specificvolume is reduced and the high pressure valve 22 is then opened (point Bof FIGS. 4 and 5), so that the pressure in chamber 23 again rises to the850 psi level of the CO₂ cylinder (point C of FIGS. 4 and 5). Thetemperature of the chamber is then again controlled so as to increasefrom 0° C. to a much higher temperature, selected in this exemplary caseas 43° C. The pressure rises from 850 psi to a much higher value showntheoretically in the ideal case as 6000 psi (point D of FIG. 4). In apractical case, the pressure must be controlled so as not to exceed thelimits imposed by the chamber 23. In a practical case, the high pressurevalue, for example, is increased to 3000 psi (point D of FIG. 5).

At point D, the CO₂ is in a super critical state and acts as asupercritical fluid. At such point, the CO₂ is supplied to the polymerplastic material 25 to form a fluid/polymer solution containing asufficient amount of supercritical CO₂ for a supermicrocellular foamingprocess. In effect the solution can be thought of as being saturatedwith supercritical CO₂, which saturation process occurs over aparticular saturation time period, depending on the thickness of thepolymer plastic. For example, if material 25 is a sheet of plasticizedpolyvinylchlorine (PVC) material, having a thickness of about 1/16 inch,a sufficient time period for such operation is about 5 minutes, suchtime being more or less dependent on the diffusion distance of thepolymer (the thickness thereof) and the diffusion characteristics of thesupercritical fluid, e.g., CO₂, in the particular polymer used.

Following formation of the desired fluid/polymer material, the chamberis opened and the material is removed therefrom so that the pressure andtemperature thereof rapidly assumes ambient room conditions (e.g., 77°F., 0.1 Mpa). Such rapid changes in temperature/pressure conditionsinduce a thermodynamic instability so that foaming (cellular nucleationand cell expansion) takes place within the material. The foaming time toachieve a desired supermicrocellular foam PVC material, for example, isabout one or two minutes, such time being more or less dependent on thetemperature of the material prior to removal. It is found that such PVCmaterial achieves a cell density of about 2×10¹² cells/cc. and anaverage cell size of about 0.8 microns. The cell density is primarily afunction of the amount of supercritical fluid in the fluid/polymersolution as measured relative to the unfoamed material. Amicrophotograph of an exemplary cross-section of such material is shownin FIG. 6, magnified 2000 times, the cell distribution beingsubstantially uniform throughout the material.

Similar experimental foamed materials were made using substantially thesame technique. For example, a glycol modifiedpolyethylene-terephthalate (i.e., copolyester) polymer material (PETG)was supplied with sufficient supercritical CO₂ fluid over a time periodof about 10 hours and, when removed to room temperature and pressureconditions, the fluid/polymer system was found to foam in about one ortwo minutes, thereby producing a supermicrocellular foamed PETG materialhaving a substantially uniform cell distribution, a cell density ofabout 3×10¹⁰ cells/cc. and an average cell size of about 5 microns. Amicrophotograph thereof is shown in FIG. 7 at a magnification of 1000times.

In some cases, particularly when using a semi-crystalline material, ithas been found that the foaming temperature must be higher than ambientroom temperature. For example, when a sheet of rigid PVC material havinga thickness of 1/16 inch (1.59 mm) is used, an effectively saturatedfluid/polymer system can occur at a pressure of about 1500 psi (10.2MPa) and a temperature of 43° C. over a time period of about 15 hours.Following the formation thereof, the material is foamed at a much highertemperature than ambient room temperature, e.g., at 160° C. at ambientpressure. Such foaming can be produced by removing the saturated rigidPVC polymer from the chamber 24 and placing it in a liquid glycerin baththe temperature of which is at the desired 160° C. levelSupermicrocellular foaming was found to take place in about 10 seconds.In such case, an average cell size of about 1.0 micron and a celldensity of about 2×10¹² cells/cc. was achieved, there being a reasonablyuniform distribution of such cells throughout the material. Amicrophotograph of such foamed rigid PVC material is shown in FIG. 8 ata magnification of 5000 X.

A similar foaming temperature 160° C. was used for both low density andhigh density polyethylene (LDPE and HDPE) polymers. In the case of a lowdensity sheet of PE having a thickness of 1/16 inch (1.59 mm), theformation of a suitable fluid/polymer system took place at a pressure of3000 psi and a temperature of 43° C. over a 10 hour time period, whilesupermicrocellular foaming occurred at the 160° C. level at ambientpressure in about 20 seconds. Such operation produced very small averagecell sizes of about 0.1 micron and cell densities of about 5×10¹⁴cells/cc. In the case of a sheet of high density PE having a thicknessof 1/16 inch (1.59 mm), formation of a desired fluid/polymer system alsooccurred at 3000 psi and 43° C. over a 10 hour time period, whilefoaming occurred at 160° C. and ambient pressure in about 20 seconds.Such operation produced very small average cell sizes of about 0.2microns and cell densities of about 6×10¹³ cells/cc. Microphotographs ofexemplary foamed LDPE polymers and foamed HDPE polymers are shown inFIGS. 9 and 10, respectively, at magnifications of 5000 X (FIG. 9) andof 2000 X (FIG. 10), respectively.

In a further exemplary case, a sheet of polycarbonate polymer having athickness of 1/16 inch was supplied with supercritical CO₂ to form asuitable fluid/polymer system at a pressure of 1500 psi (10.2 MPa) and43° C. over a 15 hour time period, while foaming occurred at 160° C. andambient pressure in about 10 seconds to produce average cell sizes ofabout 2 microns and cell densities of about 2×10¹¹ cells/cc. Amicrophotograph of an exemplary cross-section thereof is shown in FIG.11 at a magnification of 2000 X.

The bar diagrams depicted in FIGS. 12 and 13 show the correlationbetween average cell sizes and cell densities for the above discussedexemplary foamed materials. In the figures, the bars as related to eachmaterial are so designated in each case and, as can be seen, generallythe smaller the cell sizes obtained the greater the cell densities thatcan be achieved.

While the producing of a supercritical fluid for use in the process ofthe invention is performed in the above particular examples at atemperature of 43° C. and at pressures of 1500 psi or 3000 psi, suchtemperatures can range from about 35° C. to about 45° C., or higher, andsuch pressures can range from about 1400 psi to about 6000 psi, or more.The supercritical fluid should have a relatively high density, e.g., forsupercritical CO₂ fluid a density of about 0.016 moles per cubiccentimeter to about 0.022 moles per cubic centimeter can be used.

Although the technique described above with reference to FIG. 3 is ineffect a batch processing technique, foamed materials can also be madeusing a continuous process in which polymer plastic pellets or sheetsare used. FIG. 14, for example, depicts one such continuous techniqueusing a co-rotating twin screw extruder of a type well-known to those inthe art for supplying a sheet of polymer to a chamber 38 for foaming ofthe polymer using a supercritical fluid.

As seen in the diagram of FIG. 14, an extruder barrel 30 having aplurality of barrel heaters 31 has a polymer material, e.g., in the formof polymer pellets, supplied thereto via a hopper 32. Extruder barrel 30contains a co-rotating meshing twin screw assembly 33 for extrudingpolymer plastic material to a sheet die 34. A continuous sheet ofpolymer material is thereby supplied to an arrangement 36 of rollersheld at a substantially constant temperature. A motor 37 is used tocontrol the position of roller 35 so as to control in turn the residencetime of the polymer sheet in chamber 38 by controlling the length of thesheet resident in the chamber. The roller system 36 is positioned withina chamber 38 to which is supplied a supercritical fluid from a source 39thereof. For example, a source 39 of CO₂ in a gaseous form supplies CO₂gas to a compressor 40, the temperature of the gas and the pressure atthe compressor being controlled to place the CO₂ in its supercriticalstate when it is supplied to chamber 38.

As the sheet of polymer plastic travels through the roller system 36 ata selected speed, e.g., at a linear space of about 1.0 inch/second, thesupercritical fluid and the polymer form a fluid/polymer system,sufficient fluid being supplied so that the sheet is effectivelysaturated with fluid as it leaves chamber 38. The saturated sheet ofpolymer emerges from chamber 38 into a foaming chamber 41 via a suitabledynamic pressure seal 42 and thence through a pair of chilled rollers43. The drop in pressure occurring from the pressure in chamber 38 tothe pressure in chamber 41, e.g., ambient pressure, as the fluid/polymersheet exits through the dynamic seal 42 to the chilled rollers 43 causesa nucleation of cells within the fluid/polymer material which cellularnucleation is maintained at the chilled rollers 43. The fluid/polymersheet material is then heated by passing the sheet adjacent foamingheaters 44, the time of residence therethrough being controlled bychanging the length of the sheet resident in chamber 41 adjacent heaters44 using a motor 45. The increase in temperature of the fluid/polymermaterial causes the nucleated cells to expand so that the polymermaterial is appropriately foamed as it leaves the region of the foamingheaters 44.

In a further optional step, the foamed material can then be annealed,e.g., for crystallization of the foamed polymer, if desired, bysupplying the foamed sheet material to annealing heaters 46, the timefor such annealing process being controlled by changing the length ofthe sheet resident adjacent heaters 46 using a motor 47. The foamed, andannealed, material can then be supplied from foaming chamber 41 to atake-up roller device 48 for storage.

An alternative continuous foaming process is depicted in FIG. 15 usingthe system of FIG. 14 in a somewhat different manner. As can be seentherein, a supercritical fluid is supplied to a polymer plastic materialwhile the latter material is being extruded from extruder barrel 30, thesupercritical fluid, e.g., CO₂, being obtained from a CO₂ gas supply 50and a compressor 51, as before. The supercritical fluid is supplied tothe interior of heated extruder barrel 30 at an appropriately selectedposition so as to introduce the fluid into the molten polymer material.Sufficient supercritical CO₂ is supplied so as to form a moltenfluid/polymer material in which the polymer is effectively saturatedwith supercritical fluid. The molten fluid/polymer material exits fromextruder barrel 30 and is supplied to a sheet die 34. Sheet die 34 formsa sheet of such fluid/polymer material, which saturated sheet is thensupplied to an arrangement 53 of chilled rollers in a foaming chamber52. The pressure in the chamber 52 is maintained at a level lower thanthat at the extruder barrel exit and as the pressure drops upon enteringof the fluid/polymer material into chamber 52, cell nucleation occurswithin the material. The chilled rollers maintain the cell nucleationcondition and the fluid/polymer material is then supplied to foamingheaters 44, where cell expansion and, thereby, completion of the foamingprocess is achieved. As in the system of FIG. 14, the foamed polymermaterial can be annealed, e.g., for crystallization of the foamedpolymer if desired, by annealing heaters 46 (optional) and the annealedfoamed polymer material can exit the foaming chamber for supply to atake-up device 48 via chilled rollers 54. Motors 37, 45 and 47 are usedas above, to control the residence times of the sheet at thecorresponding regions of chamber 52.

A further alternative embodiment of the continuous process shown inFIGS. 14 and 15 is depicted in FIG. 16, wherein a supercritical fluid,e.g., CO₂ in its supercritical state, is supplied to an extruder barrel30, as in FIG. 15, for providing saturated extruded fluid/polymermaterial therefrom. The extruded material is then formed into a sheet offluid/polymer material and supplied to a pressurized chamber 55, thepressure in which is suitably controlled by a pressure controller 59.The sheet material is supplied to an arrangement 56 of constanttemperature rollers and thence exits chamber 55 via a dynamic pressureseal 57.

If the pressure in chamber 55 is maintained at substantially the samepressure as the saturation pressure of the supercritical fluid suppliedby compressor 51, both cell nucleation and cell expansion occur as thefluid/polymer sheet exits via dynamic seal 57 due to the pressure dropfrom the pressure in chamber 55 to the lower pressure in an annealingchamber 58. The foamed polymer material is then passed through chilledrollers 60 to maintain its foamed condition and supplied to annealingheaters 46 and thence to take-up device 48, as before. Residence timesin chambers 55 and 58 are controlled by motors 37 and 47, respectively,as before.

If the pressure in chamber 55 is controlled to be at a level below thatof the saturation pressure of the supercritical fluid supplied bycompressor 51, cell nucleation occurs as the sheet material exits sheetdie 34 into the lower pressure chamber 55. The chilled rollers 56maintain the nucleated cells. Cell expansion then occurs as the polymermaterial exits at dynamic seal 57 to an even lower pressure annealingchamber 58, e.g., at ambient pressure, so that the completely foamedpolymer material is obtained at that point. The chilled rollers 60maintain the cell expansion. In such an operation as depicted in FIG.16, foaming (i.e., cell nucleation and cell expansion) takes placesubstantially solely due to the pressure differentials which occur inthe system. Such operation can be contrasted with that of FIG. 14, forexample, wherein cell nucleation occurs due to the pressure differentialat dynamic seal 42 and cell expansion occurs due to the temperaturedifferential at foaming heaters 44. Such operation can also becontrasted with that of FIG. 15, for example, wherein cell nucleationoccurs due to the pressure differential at the exit of sheet die 34 andcell expansion occurs due to the temperature differential at foamingheaters 44.

The embodiments discussed with reference to FIGS. 1-16 disclosetechniques in which foaming can take place, using supercritical fluids,at various temperatures, i.e., at room (ambient) temperature or athigher temperatures. FIG. 17 depicts an exemplary system in which thefoaming operation and the forming of an article therefrom can beaccomplished in the same overall operation at ambient, or room,temperature. As can be seen therein, a mold comprising a lower mold body61 having a mold cavity 62 and an upper mold body 63 shaped to conformto the mold cavity 62 are arranged so that mold body 61 is fixedlymounted within a chamber 64 and mold body 63 is movably mounted to movereciprocally into and out of cavity 62 using a suitable externallyapplied hydraulic jack or piston force, as shown by arrow 72, via asuitable dynamic pressure seal 65. A pliable sheet 66 of a polymerplastic material is mounted above cavity 62 of mold body 61 between twosuitably shaped holders 67 so that, when mold body 63 is moveddownwardly into cavity 62, a cup-shaped article of polymer plasticmaterial can be formed therebetween. Prior to forming the article, asupercritical fluid, e.g., CO₂ in its supercritical state, from a source68 thereof, is supplied to chamber 64 via a suitable valve 69, thesupercritical fuid normally being supplied at a temperature higher thanambient temperature. Chamber 64 is pressurized to a relatively highpressure, e.g., 3000 p.s.i. (PMa), the temperature within chamber 64,however, being maintained at a suitable temperature on the order of thecritical temperature, or higher, of the supercritical fluid. Thesupercritical fluid in effect saturates the polymer sheet 66 after atime period which depends on the polymer material involved. Thetemperature in chamber 64 is reduced to room (ambient) temperature and,when the polymer sheet is saturated with supercritical fluid, the moldbody 63 is moved downwardly into cavity 62 and, preferably, the pressurein the chamber is then reduced via the operation of pressure reliefvalve 70. The drop in pressure causes a cell nucleation and cellexpansion within the polymer material as the molding of the articleoccurs, thereby causing a foaming of the polymer material and theforming of an article from the foamed material, the article having asupermicrocellular structure. Accordingly, the article is both foamedand formed at room (ambient) temperature in one overall operation.

In the above disclosed embodiments, there is a finite time which isrequired for a polymer material to become saturated with a supercriticalfluid. i.e., for a sufficient amount of supercritical fluid to beintroduced into the polymer to form a fluid/polymer solution which canbe appropriately foamed to provide a desired supermicrocellular foamedmaterial. While in some cases such time can be as low as 10 minutes, e g, when using a soft PVC material having a thickness of 1/16 inch (1.59mm), in other cases longer times may be required depending on thethickness desired. While such embodiments can be useful in manyapplications, in other applications it may be desirable to reduce thetime need for such purpose. For example, in order to enhance the abilityto use the technique of the invention in some applications to achieverelatively high production rates for obtaining supermicrocellular formedmaterial, it is often desirable to use much shorter saturation timeperiods. One technique for doing so is depicted in the system shown inFIG. 18 in which a supercritical fluid is introduced into an extrusionbarrel 70, for example, for injecting the saturated material into amold.

As can be seen in the diagrammatic presentation of FIG. 18, an extrusionbarrel 70 utilizes a screw 71, with integrated mixing elements 83, of atype having irregular blades, as would be well known to those in theart, into which plastic pellets of a polymer material are introduced viaa hopper assembly 72. The extrusion barrel is heated so that the pelletsbecome plasticized and reach a molten state as they are moved by themixing screw along the barrel 70, in a manner similar to that discussedwith reference to FIGS. 14, 15 and 16. A source 82 of CO₂ gas isintroduced into the extrusion barrel at a selected position along mixingscrew 71 via the operation of a suitable flow control valve 73, thetemperature and pressure in the extrusion barrel at that point beingcontrolled so as to be greater than the critical temperature andpressure for converting the CO₂ in gaseous form into CO₂ in itssupercritical state. The CO₂ gas may be preheated before insertion, ifdesired, to prevent too sudden an increase in pressure in the barrel atthe higher temperature of the barrel. Alternatively, the CO₂ gas can beconverted to its supercritical state externally to the extrusion barreland supplied to the mixing screw as a supercritical CO₂ fluid.

The supercritical CO₂ fluid is mixed with the molten polymer material bythe mixing screw and such mixing enhances the subsequent diffusion into,and effective saturation of supercritical CO₂ fluid in, the polymerbecause the contact area of the two materials being mixed is increasedby the mixing process and the depth required for diffusion is decreasedthereby.

Thus, the supercritical CO₂ fluid is mixed with the molten polymer bythe motion of the mixing screw to aid in forming a solution. As themixing screw rotates, it generates a two-dimensional shear field in themixed CO₂ /polymer system. The bubbles of supercritical CO₂ fluid in thepolymer melt are stretched along the shear directions of the shearfield. The stretched bubbles are then broken into smaller sphericalshaped bubbles by the perturbation of the laminar flow which isgenerated by the mixing screw. The irregular blades used in the mixingscrew change the orientation of the CO₂ /polymer interface relative tothe streamlines, which change increases the efficiency of the laminarmixing occurring therein.

The CO₂ /polymer mix is supplied to a static mixer 74 which continuallychanges the orientation of the CO₂ /polymer interface relative to thestreamlines and thereby also enhances the mixing process. Static mixersfor use in an extrusion barrel are well known to the art and are madeand sold commercially. The diameter of static mixer 74 should be smalland the static mixer can comprise a selected number of mixer elements75, as further discussed below.

If the diameter of the static mixer elements is too large, the flow rateof the CO₂ /polymer mixture therethrough is small and, consequently, theshear field generated by the static mixer elements is small. Thespherical shapes of the bubbles would thereby be maintained because thesurface tension would be dominant and, in effect, the surface tensionwould overcome the effect of the relatively small shear field. When theflow rate is too small, a static mixer is not effective for mixing theCO₂ /polymer system into a solution because of such dominant surfacetension. Hence, it is desirable to make the diameter of the static mixerrelatively small.

The characteristic length of the static mixing which occurs in staticmixer 74, i.e., the striation thickness of the mixed CO₂ /polymerlayers, is approximately d/2^(n) where d is the diameter of the staticmixer elements and n is the number of the mixing elements 75. Bettermixing occurs when mixer elements having a small radius are used becausesuch characteristic length of the mixing decreases as the diameterdecreases, as well as when a relatively large number of mixing elementsis used. The number of mixing elements and the diameters thereof can beselected so as to assure a satisfactory and adequate static mixingoperation.

During the static mixing of the CO₂ /polymer system, the CO₂ moleculesin the bubbles also tend to diffuse somewhat into the polymer meltmaterial which surrounds each bubble. However, the primary diffusionoperation takes place in a diffusion chamber 76 into which the two-phasegas/polymer mixture is introduced. The mixture then becomes a completesingle-phase solution in the diffusion chamber as the CO₂ diffuses intothe polymer therein. The CO₂ concentration in the single-phase CO₂/polymer solution thereby produced is substantially uniform throughoutthe solution and the solution is effectively homogeneous. If thesupercritical CO₂ fluid does not diffuse into and saturate the polymeruniformly and homogenously, the foamed structure that is ultimatelyformed will not be uniform because the cell morphology strongly dependson the local gas concentration in the solution.

The homogeneous and uniform fluid/polymer solution in diffusion chamber76 is then heated in a heating section 77 thereof where the solution israpidly heated (in a typical case the temperature may rise from about190° C. to about 245° C., in about 1.0 second, for example), so as toform nucleated cells in the saturated solution due to the thermodynamicinstability which is created because of the decreased solubility of thefluid/polymer solutions at the higher temperature. The greater thedecrease in solubility which occurs, the higher the cell nucleation rateand the larger the number of cells nucleated. To prevent the nucleatedcells from growing in the extrusion barrel 70 a high barrel presssure ismaintained. The solution with nucleated cells is then injected into amold cavity 78 of a mold 79, the pressure in the mold cavity beingcontrolled by providing a counter pressure to prevent cell growth duringthe mold filling process. The counter pressure is provided by theinsertion of air under pressure from a source 80 thereof via shut-offvalve 81. Finally, cell growth occurs inside the mold cavity when themold cavity is expanded and the pressure therein is reduced rapidly,thereby producing a pressure instability which enhances cell growth.

Accordingly, expansion of the mold provides a molded and foamed articlehaving the small cell sizes and high cell densities desired. By using amixing screw for providing a shear field which produces a laminar flowof the mixed materials and then by using both a static mixer havingsmall diameter mixing elements and a selected number of such mixingelements and a diffusion chamber, saturation of the polymer materialwith supercritical CO₂ fluid occurs. The time period required to providesuch saturation can be reduced from that required in the embodiments ofthe invention discussed previously so that it is possible to achievecontinuous operation at relatively high production rates that would notbe possible when longer saturation times are needed.

The provison of extremely small cell sizes and high densities thereof ina foamed polymer material, as achieved when using supercritical fluidsto provide the foaming operation, as described with reference to theabove embodiments of the inventions brings about substantially improvedproperties for the foamed materials obtained, particularly compared withprevious standard cellular or microcellular foamed materials. Thus, themechanical strengths and toughness thereof are substantially greater,even when the weight of the material (i.e., the material density) isconsiderably reduced. Moreover, less polymer material is used in theprocess and, accordingly, material is conserved and the costs thereofare reduced.

While the embodiments of the invention described above representpreferred embodiments thereof, modifications thereof and still otherembodiments may occur to those in the art within the spirit and scope ofthe invention. Hence, the invention is not to be construed as limited tothe specific embodiments thereof described above, except as defined bythe appended claims.

What is claimed is:
 1. A system for producing a foamed materialcomprisingextrusion means means connected to said extrusion means forsupplying a material to be foamed to said extrusion means; die means forshaping material; said extrusion means connected to said die means forproviding extruded material to said die means; means for heating saidextrusion means to cause said extrusion means to provide said extrudedmaterial at a higher temperature than room temperature to permit saiddie means to produce shaped continuous heated material; means forengaging and transporting said shaped continuous heated material throughan enclosed volume; means connected to said enclosed volume forsupplying supercritical fluid to said enclosed volume to introduce saidsupercritical fluid into said shaped continuous heated material at ahigher pressure than atmospheric pressure; means for retaining saidshaped continuous heated material within said enclosed volume for asufficient time period to permit said supercritical fluid to saturatesaid shaped continuous heated material; means for removing said shapedcontinuous heated material saturated with said supercritical fluid fromsaid enclosed volume at a pressure which is less than said higherpressure; and foam heating means for heating said material which hasbeen removed from said enclosed volume so as to produce shapedcontinuous heated foamed material having a plurality of cellsdistributed substantially throughout said foamed material.
 2. A systemin accordance with claim 1 wherein said die means is a sheet die meansfor providing a continuous sheet of heated material.
 3. A system inaccordance with claim 2 wherein;said engaging and transporting meansincludes a plurality of rollers maintained at a substantially constanttemperature for transporting said continuous sheet of heated materialthrough said enclosed volume; and said retaining means including meansfor controlling the position of at least one of said rollers so as tocontrol the residence time of said continuous sheet of heated materialas it is transported through said enclosed volume.
 4. A system inaccordance with claim 2 wherein said removing means includes a dynamicpressure seal through which said heated sheet material is removed fromsaid enclosed volume and further including chilled roller means engagingand transporting said removed heated sheet material from said enclosedvolume at a temperature below said higher temperature.
 5. A system inaccordance with claim 2 wherein said foam heating means includes atleast one heater and heater transporting means engaging said removedsheet material for transporting said removed sheet material along a pathadjacent said heaters.
 6. A system in accordance with claim 5 whereinsaid heater transporting means includes:at least one roller; and meansfor controlling the position of at least one of said rollers to controlthe time over which said sheet is transported along said path adjacentsaid heaters.
 7. A system in accordance with claim 2 and furtherincluding means for further engaging said removed continuous sheet ofheated foamed material for annealing said continuous sheet of heatedfoamed material.
 8. A system for providing a foamed materialcomprisingextrusion means; die means connected to said extrusion meansfor shaping material; means connected to said extrusion means forsupplying a material to be foamed to said extrusion means; means forheating said extrusion means to place said material into a molten stateduring the extrusion thereof in said extrusion means; means connected tosaid extrusion means for supplying a supercritical fluid at a higherpressure than atmospheric pressure to said extrusion means to introducesaid supercritical fluid into said molten material so that said materialis saturated with said supercritical fluid to form a solution, saidsupercritical fluid saturated material being supplied from saidextrusion means to said die means to produce a shaped continuousmaterial saturated with said supercritical fluid; means for engaging andtransporting said shaped continuous material through an enclosed volumehaving a pressure which is lower than said higher pressure so as toproduce cell nucleation in said shaped continuous material and means formaintaining the temperature of said shaped continuous material at aselected temperature as said material is transported through saidenclosed volume at said lower pressure; and foam heating meanscomprising heaters for heating said shaped continuous material as itexits said enclosed volume so as to produce a shaped continuous foamedmaterial having a plurality of cells distributed substantiallythroughout said shaped continuous foamed material.
 9. A system inaccordance with claim 8 wherein said die means is a sheet die means forproviding a continuous sheet of material.
 10. A system in accordancewith claim 9 wherein said transporting and temperature maintaining meansincludes a plurality of chilled rollers.
 11. A system in accordance withclaim 10 wherein said engaging and transporting means further includesmeans for controlling the position of at least one of said rollers so asto control the residence time of said continuous sheet of material as itis transported through said enclosed volume.
 12. A system in accordancewith claim 9 wherein said foam heating means includes:at least oneroller engaging and transporting said continuous sheet of materialexiting said enclosed volume along a path adjacent said heaters; andmeans for controlling the position of at least one of said one or morerollers to control the time over which the continuous sheet of materialis transported along said path adjacent said heaters.
 13. A system inaccordance with claim 9 and further including means further engagingsaid continuous sheet of foamed material for annealing said continuoussheet of foamed material.
 14. A system for producing a foamed materialcomprisingextrusion means; die means connected to said extrusion meansfor shaping material; means for supplying a material to be foamed tosaid extrusion means; means for heating said extrusion means to placesaid material in a molten state during extrusion thereof in saidextrusion means; means connected to said extrusion means for supplying asupercritical fluid at a higher pressure than atmospheric pressure tosaid extrusion means to introduce said supercritical fluid into saidmolten material so that said molten material is effectively saturatedwith said supercritical fluid, said material which is saturated withsaid supercritical fluid being supplied from said extrusion means tosaid die means to produce a shaped continuous material; means forengaging and transporting said shaped continuous material through anenclosed volume having a pressure which is substantially the same assaid higher pressure and means for maintaining the temperature of saidshaped continuous material at a selected temperature as said shapedcontinuous material is transported through said enclosed volume at saidhigher pressure; and means for reducing the pressure and the temperatureof said shaped continuous material as it exits from said enclosed volumeso as to produce a shaped continuous foamed material having a pluralityof cells distributed substantially throughout said shaped continuousfoamed material.
 15. A system in accordance with claim 14 wherein saiddie means is a sheet die means for providing a continuous sheet ofmaterial.
 16. A system in accordance with claim 15 wherein saidtransporting and temperature maintaining means includes a plurality ofrollers.
 17. A system in accordance with claim 16 wherein said engagingand transporting means further include means for controlling theposition of at least one of said rollers to control the residence timeof said continuous sheet of material as it is transported through saidenclosed volume.
 18. A system in accordance with claim 15 and furtherincluding at least one chilled roller positioned near the exit of saidenclosed volume through which said continuous sheet of foamed materialpasses so as to maintain the foamed condition thereof.
 19. A system inaccordance with claim 15 and further including means further engagingsaid continuous sheet of foamed material for annealing said continuoussheet of foamed material.
 20. A system for producing a foamed materialcomprisingextrusion means; die means connected to said extrusion meansfor shaping material; means for supplying a material to be foamed tosaid extrusion means; means for heating said extrusion means to placesaid material in a molten state during extrusion thereof in saidextrusion means; means connected to said extrusion means for supplying asupercritical fluid at a higher pressure than atmospheric pressure tosaid extrusion means to introduce said supercritical fluid into saidmolten material so that said molten material is substantially saturatedwith said supercritical fluid, said saturated molten material beingsupplied from said extrusion means to said die means to produce a shapedcontinuous material; means for engaging and transporting said shapedcontinuous material through a first enclosed volume having acontrollable pressure which is lower than said higher pressure toproduce cell nucleation in said shaped continuous material and means formaintaining the temperature of said shaped continuous material at aselected temperature to maintain said cell nucleation as said shapedcontinuous material is transported through said first enclosed volume atsaid lower pressure; and means for engaging said shaped continuousmaterial as it exits from said first enclosed volume and fortransporting said exiting material into a second enclosed volume havinga pressure lower than said controllable pressure so as to produce ashaped continuous foamed material having a plurality of cellsdistributed substantially throughout said shaped continuous foamedmaterial.
 21. A system in accordance with claim 20 wherein said diemeans is a sheet die means for providing a continuous sheet of material.22. A system in accordance with claim 20 wherein the pressure in saidsecond enclosed volume is at atmospheric pressure.
 23. A system inaccordance with claim 20 and further including means for controlling theresidence times of said shaped continuous material in said first andsecond enclosed volumes.
 24. A system for producing a foamed materialcomprisinga barrel; a screw member mounted for rotation within thebarrel and having a plurality of irregular blades positioned on saidscrew member; means for introducing a material to be foamed into saidbarrel for movement along said barrel toward said irregular blades bysaid screw member; means for heating said barrel to place said materialinto a molten state; means for introducing a supercritical fluid intosaid barrel at said irregular blades at a temperature and pressure abovethe critical temperature and pressure of said supercritical fluid formixing said fluid with said molten material to provide a mixturethereof; a static mixer for receiving said mixture and for changing theorientations of the interfaces between said material and saidsupercritical fluid in the mixture; a diffusion chamber for receivingsaid mixture from said static mixer to diffuse the supercritical fluidinto the material to be foamed, said static mixture and diffusionchamber providing to provide a solution of said material substantiallysaturated with said fluid and having a substantially uniformconcentration of fluid throughout said solution; means for rapidlyheating said solution to provide a plurality of nucleated cells in saidsolution at a pressure which prevents expansion of said cells in thesolution; means for receiving said solution from said diffusion chamberand for expanding the cells in said solution to provide a foamedmaterial.
 25. A system in accordance with claim 24 wherein saidreceiving means includesa mold for receiving said solution from saiddiffusion chamber and having a counter pressure for initially preventingexpansion of said cells in the solution; and means for subsequentlyrapidly reducing the counter pressure in said mold to expand the cellsin said solution to provide a molded foamed article in said mold.